Aluminium nitride ceramics and method for preparing the same

ABSTRACT

Aluminium nitride ceramics improved in heat radiation property used as a substrate for integrated circuits and package material, comprising a sintered article consisting mainly of aluminium nitride and having a thermal conductivity higher than 100.W/m.K at room temperature and a surface layer consisting mainly of aluminium nitride or oxide glass deposited on the sintered article. 
     A paste of aluminium nitride powder or oxide glass powder is coated on a surface of the sintered article of aluminium nitride and then is sintered to prepare a dense smooth surface layer.

This application is a divisional of application Ser. No. 08/958,873,filed Oct. 27, 1997, now U.S. Pat. No. 5,955,148 which is a continuationof Ser. No. 08/439,099, filed May 11, 1995, now abandoned which is adivisional of Ser. No. 08/210,502, filed Mar. 18, 1994, refiled as acontinuation in application Ser. No. 08/698,293, filed Apr. 15, 1996 nowU.S. Pat. No. 5,677,052 issued Oct. 14, 1997 now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to aluminium nitride ceramics havingimproved heat-radiation property which can be used as a variety ofelectronics parts including a substrate for integrated circuits andpackaging material.

2. Description of the Related Art

Aluminium nitride ceramic (or AlN ceramics, hereinafter) or a sinteredarticle of aluminium nitride is obtained by sintering a powder materialconsisting mainly of aluminum nitride. This aluminium nitride ceramicpossesses good insulation property, high mechanical strength and highthermal conductivity and is easy to be bonded to metal conductors andhence is expected to be used as a substrate for integrated circuits andpackaging material.

When the AlN ceramic is used as a substrate for integrated circuits andpackaging material, a metallized layer is usually deposited on a surfaceof the AlN ceramic, because electronic components such as transistor,diode, IC and LSI can not be mounted directly on the AlN ceramic. Suchmetallized layer is required also when a circuit pattern is printed onthe surface of AlN ceramic.

In order to improve the thermal conductivity of AlN ceramics, it is ausual practice and an effective technique to grow crystalline particlesof AlN so that grain boundaries are reduced or eliminated. However, asintered article of AIN whose crystal particles are grown often possessvoids or defects caused by lack of crystal particles, although itsthermal conductivity is improved. A metallized layer deposited on such adefective surface of sintered article of AlN does not possess enoughstrength at an interface between the metallized layer and the sinteredarticle of AlN and will be a cause of such troubles as breakage incircuit patterns and increment of resistivity in wiring lines.

Since two requirements to improve thermal conductivity and to improvesurface smoothness are contradictory or trade-off, it is impossible toproduce AIN ceramics possessing high thermal conductivity as well ashigh surface smoothness simultaneously by prior arts.

An object of the present invention is to solve the problems of knowntechniques and to provide AlN ceramics possessing high thermalconductivity as well as high surface smoothness and methods forproducing the same.

SUMMARY OF THE INVENTION

The present inventors found that the above-mentioned contradictory tworequirements to improve thermal conductivity and to improve surfacesmoothness can be satisfied simultaneously by depositing a dense smoothsurface layer on a surface of a sintered article of AlN.

The present invention provides a aluminum nitride ceramic comprising asintered article consisting mainly of aluminium nitride and having athermal conductivity higher than 100 W/m.K at room temperature and asmooth dense surface layer formed on a surface of the sintered articleand having a surface roughness (Ra) of lower than 0.3 μm, no defectlarger than 25 μm being found on a surface of the surface layer.

BRIEF DESCRIPTION OF THE DRAWING

The aluminum nitride ceramic according to the present invention has astructure shown in a cross section of FIG. 1. A smooth dense surfacelayer (2) is formed on a surface of a sintered article (1) consistingmainly of aluminium nitride and having a thermal conductivity higherthan 100 W/m.K at room temperature. The surface layer (2) has a surfaceroughness (Ra) of lower than 0.3 μm and no defect larger than 25 μm isfound on a surface of the surface layer (2).

Preferable thickness of the surface layer (2) is 10 to 250 μm. Particlesor grains of which the aluminium nitride sintered article (1) is madehave preferably particle sizes of larger than 2 μm. Preferably, thesurface layer (2) consists mainly of aluminium nitride or oxide glass.In case of oxide glass is used as the surface layer (2), oxide glasspossess preferably a thermal expansion coefficient of 3.0 to 6.0 ppm/°C.The surface layer may have a multi-layered structure consisting of aplurality of oxide glass layers. Differences in softening points ofoxide glass layers are preferably larger than 5° C. Preferably, each ofthe oxide glass layers has a thickness of of 1 to 100 μm. At least thefirst oxide glass layer deposited directly on a surface of the sinteredarticle is preferably made of oxide glass which do not contain Na, K, Rband Pb.

The present invention provides also a method for producing aluminiumnitride ceramic having a dense smooth surface layer possessing a surfaceroughness (Ra) of lower than 0.3 μm and possessing no defect larger than25 μm on its surface, characterized by the steps of applying a paste forsaid surface layer onto a surface of a sintered article or a nonesintered preform of aluminium nitride and sintering said paste togetherwith said preform if the later exists.

The resulting surface layer is further polished optionally aftersintering. The paste contain preferably more than 50% by weight ofaluminium nitride powder or oxide glass powder. Oxide glass possesspreferably a thermal expansion coefficient of 3.0 to 6.0 ppm/°C. Thesurface layer can be prepared by repeating two steps of applying anoxide glass paste onto a sintered article of AIN and sintering the oxideglass paste for more than one time, or by effecting sintering of alloxide glass paste layers formed on a sintered article at one time. Atleast the first oxide glass layer deposited directly on a surface of thesintered article is preferably made of oxide glass which do not containNa, K, Rb and Pb. Preferably a surface of the sintered article ofaluminium nitride is oxidized before the paste of oxide glass is coated.The oxidation is carried out preferably at a temperature between 200 and1,300° C.

The content of AlN in the sintered article of AlN is preferably morethan 80% by weight and more preferably more than 93% by weight in orderto obtain high thermal conductivity.

The sintered article of AIN can contain the other components such as theconventional sintering aids of IIa and IIIa group elements, knowncoloring agents of IVa, Va and VIa group elements, oxygen, boron, carbonor the like in addition to inevitable impurities such as Fe, Si etc.

The IIa and IIIa group elements for sintering aids can present in a formof their oxides, fluorinated oxides and aluminate in the sinteredarticle of AlN or more than one compound selected from a groupconsisting of oxides, carbides and nitrides of IIa group elements and/orIIIa group elements and compounds which will produce them when thecompounds are sintered can be added. The proportion of IIa and IIIagroup elements or their compounds is 0.01 to 10% by weight, preferably0.1 to 5% by weight in term of element. If this proportion become lowerthan 0.1%, it is difficult to obtain dense fine sintered article of AlNbecause so-called the packaging or densification effect by sintering aidcan not be expected. On the contrary, if the proportion exceeds 5%,excess liquid phase oozes out of a surface of the sintered article,resulting in a trouble of unstable spotted color tone on the finalsintered product.

When a black-colored sintered article of AIN is desired, compounds ofIVa, Va and VIa group elements which are known as coloring agents can beadded to a material for the sintered article of AlN. The IVa, Va and VIagroup elements can be added in a form of compound(s) selected from agroup consisting of oxides, carbides, nitrides, borides and compoundswhich will produce them by sintering. It is preferable to disperse thecompounds of IVa, Va and VIa group elements as fine and uniform aspossible in order to improve the coloring effect. The coloring effectbecome remarkable when the particle size of crystal particles or grainsof AlN becomes larger than 1.0 μm and in particular, in a range of 2 to15 μm.

The “particle size” means, in this specification, an average of themaximum dimensions of particles (usually, more than 30 particles) when acut or slit section of a sintered article of AlN is observed by ascanning electron microscope (SEM) or the like.

If the particle size of crystal particles of AlN exceeds 15 μm, coloringbecome difficult with little improvement in the thermal conductivity.

The ratio of the particle size of crystal particles of AlN to theparticle size of the compound of IVa, Va and VIa elements is preferablyselected higher than 2.0 so as to improve both of thermal conductivityand coloring property.

In a sintered article of AIN, oxygen present mainly in two forms of (1)aluminate resulting from a reaction with a sintering aid and (2)aluminium oxide-nitride in which oxygen dissolved in sintered particlesas solid-solution. The oxygen content after an amount of oxygenconverted to aluminates is deducted from the total oxygen in thesintered article of AlN is preferably reduced to lower than 2% byweight, more preferably lower than 1% by weight. If the oxygen contentincrease, thermal conductivity of the sintered article of AlN becomelower. In fact, if the oxygen content becomes higher than 2% by weight,the thermal conductivity become low or unstable.

The carbon content in the sintered article of AlN is preferably in arange from 0.005 to 0.5% by weight. If the carbon content becomes lowerthan the minimum value of this range, it is impossible to produce asintered article of AlN having high thermal conductivity. On the otherhand, if the carbon content exceeds the maximum value of this range,liquid aluminates produce with the sintering aid are chemically reducedin the sintering stage, resulting in that transfer of materialsaccompanied with the liquid phase is disturbed. As a result, it isimpossible to produce a dense fine sintered article of AlN.

The boron content in the sintered article of AlN is preferably less than1% by weight. Increment of the boron content higher than 1% by weightmake it impossible to produce a precise dense fine sintered article ofAlN and cause deterioration of thermal conductivity.

The surface layer for the AlN ceramics according to the presentinvention has preferably a thickness between 10 μm to 250 μm. If thesurface layer becomes thinner than the minimum thickness, defects orvoid on a surface of the sintered article can not be repaired completelyor satisfactorily, so that it is impossible to realize a surface layerhaving the surface roughness (Ra) of lower than 0.3 μm and possessing nodefect larger than 25 μm. On the contrary, if the thickness exceeds themaximum value of this range, the surface layer becomes uneven so that itis impossible to to realize a surface layer having the surface roughness(Ra) of lower than 0.3 μm and possessing no defect larger than 25 μm.

In the first method for preparing aluminium nitride ceramics accordingto the present invention, a paste of aluminium nitride powder or oxideglass powder is prepared and applied onto a surface of the sinteredarticle of AlN and then is sintered to form a sintered surface layerwhich can be polished thereafter optionally.

In the second method for preparing aluminium nitride ceramics accordingto the present invention, the paste for the surface layer is applied toa non-sintered aluminium nitride body and both of the paste and thenon-sintered aluminum nitride body are sintered simultaneously. Theresulting sintered surface layer is polished optionally.

When a powder of oxide glass is used, a surface of the sintered articleof AlN can be polished and/or subjected to an oxidation treatment beforethe paste for the surface layer is applied. The oxidation treatmentincrease the wettability between the glass and the sintered article ofAlN.

The oxidation treatment can be effected by burning in anoxygen-containing atmosphere or water-containing atmosphere and/or bytreatment in an aqueous alkaline solution. The burning inoxygen-containing atmosphere is effected preferably under a partialpressure of oxygen gas above 100 ppm, preferably above 500 ppm. Theburning in water-containing atmosphere is effected preferably at a dewpoint of higher than −10° C., preferably above 0° C. The burningtemperature is preferably lower than 1500° C., preferably between 200and 1,300° C. If the burning temperature is lower than the range,oxidation proceed too slowly and sufficient oxidation can not beexpected. On the contrary, if the burning temperature become higher than1500° C., a surface of the sintered article cracks due to excessivedifference in thermal expansion between the sintered article of AlN andan oxide layer (alumina layer) which is produced rapidly on wholesurface of the sintered article of AlN, so that it is difficult toobtain a smooth surface layer.

The treatment in aqueous alkaline solution is preferably effected at apH higher than 10 and at liquid temperature of higher than 30° C. toaccelerate the oxidation speed.

If the partial pressure of oxygen and the dew point are outside theabove-mentioned limit, sufficient oxygen can not be supplied to thesurface of the sintered article of AlN, so that advantage of oxidationtreatment can not be expected.

The content of aluminium nitride powder or oxide glass powder in thepaste for surface layer is preferably more than 50% by weight. If thecontent of aluminium nitride powder or of oxide glass powder is lessthan 50% by weight, it is impossible to form a fine dense surface layer.

When aluminium nitride powder is used in the paste for surface layer, itis preferable to use AlN powder having a particle size less than 10 μm,preferably less than 5 μm or powder produced by pulverizing a sinteredbody of AlN. In particular, AlN powder having a particle size lower than5 μm is advantageous due to its improved sintering property. A densesmooth surface layer is difficult to be realized when AlN powder has aparticle size larger than 10 μm. The powder having particle size oflower than 5 μm can be prepared by any method including a directnitration method and a reductive nitridation method. In any case, theoxygen content in AlN powder is preferably lower than 2% by weight.

When oxide glass powder is used in the paste for surface layer, it ispreferable to use an oxide glass powder having a particle size less than50 μm or a pulverized product of the oxide glass. In particular, apowder having particle size less than 20 μm is preferably due to itsexcellent sintering property and a powder having a particle size of lessthan 10 μm is more preferable. The oxide glass powder preferably do notcontain Na, K Rb and Pb. Since these elements are very reactive with thesintered product of AlN, a gas such as nitrogen gas and NO_(x) isproduced when the surface layer is sintered, resulting in that formationof a fine dense smooth surface is disturbed.

The thermal expansion coefficient of the oxide glass is preferably in arange of 3.0 to 6.0 ppm/°C. The thermal expansion coefficient means themean thermal expansion per unit temperature from a room temperature to asintering temperature of the surface layer of oxide glass. If thethermal expansion coefficient of oxide glass is out of this range, thedifference in thermal expansion between the sintered article of AlN andthe surface layer of oxide glass becomes such big that cause crack andbreak of the sintered article of AlN and of the surface layer of oxideglass.

The smooth surface layer.can be realized much easily by depositing theoxide glass layer in a structure of multi-layers. In fact, in order torealize a smooth surface, it is necessary to adjust a composition ofoxide glass and to control sintering conditions, because components ofthe oxide glass react with aluminum nitride of the the sintered articleof AlN, even if oxide glass free from Na, K, Rb and Pb is used,resulting in that the surface layer is deformed.

This problem can be solved by adopting the structure of multi-layers ofoxide glass and by depositing the first oxide glass layer to becontacted with the sintered article of AlN whose softening point ishigher than those of the other oxide glass layers. This solutionfacilitates the adjustment of oxide glass composition and control ofsintering conditions. It is preferable to deposit a plurality of layersof oxide glass each whose softening point become lower gradually along adirection away from the sintered article in order to absorb thermalstress.

The thickness of each oxide glass layer ranges from 1 to 100 μm,preferably from 5 to 10 μm. A oxide glass layer of less than 1 μm may bedifficult to be realized as an uniform layer so that it is difficult torealize the surface roughness (Ra) of lower than 0.3 μm of the surfacelayer.

The multi-layer structure of oxide glass layers can be realized byrepeating a cycle of coating of the oxide glass paste on the sinteredarticle and sintering thereof for several times. In this case, a surfaceof oxide glass is preferably polished after every sintering operation soas to improve the surface roughness.

Or, the multi-layer structure of oxide glass layers can be realized byforming a plurality of oxide glass paste layers successively and thensintering all the oxide glass paste layers simultaneously.

The paste for surface layer may contain, as a sintering aid, compoundsof IIa and/or IIIa group elements which promote sintering reaction ofAlN particles. These compounds can be selected from a group consistingof oxides, carbides, nitrides and compounds which will produce themafter sintering. The proportion of the compounds of Ia and/or IIIa groupelements is preferably lower than 10 weight parts in term of elementwith respect to 100 weight parts of AlN. If the proportion exceeds 10weight parts, excessive liquid phase ooze out of a surface of thesintered product, resulting in such a problem that the sintered productis colored with unstable spotted tone. When a colored surface layer,such as black surface layer is required, IVa, Va and VIa groupelement(s) can be contained in the surface layer, as a coloring agent.The IVa, Va and VIa group elements can be added in a form of at leastone of compounds selected from a group consisting of oxides, carbides,nitrides and other compounds which will produce them after sintering. Inorder to improve the coloring effects, it is preferable to divide theIVa, Va and VIa group elements as fine as possible so as to dispersethem uniformly.

The paste for the surface layer is produced by adding a solventoptionally together with an organic binder to the inorganic components.The solvent may be alcohols such as ethyl alcohol and terpineol, ketonssuch as methyl ketone, esters such as dibutylphthlate, butyl carbitolacetate and water. The organic binder can be added optionally tomaintain the strength of a coated paste film. The organic binder isselected according to the solvent considered and may be cellulose-typeresins such as ethyl cellulose and nitrocellulose, acrylic resins suchas polymethyl methacrylate (PMMA) and vinyl resins such as polyvinylalcohol (PVA) and polyvinylbutylal (PVB). A surface-active agent alsocan be to added so as to improve the leveling of coated paste layer.

The coated paste for surface layer is then sintered so as to increasethe density of the surface layer. The sintering is effected preferablyat a temperature from 1,500 to 2,100° C. Sintering of lower than 1,500°C. is not economical because the sintering speed become too slow. On thecontrary, if the sintering is effected at higher than 2,100° C., AlN isdecomposed and volatilized seriously, so that it is difficult to obtaina dense fine sintered article.

When oxide glass powder is used in the surface layer, the sintering iseffected preferably at a temperature from 500 to 1,200° C. Sinteringtemperature of lower than 500° C. is not sufficient to soften glasscomponents so that it is difficult to realize a dense fine surfacelayer. On the contrary, if the sintering temperature become higher than1,200° C., reactions between AlN and glass components become intensiveto generate nitrogen gas and NO₂ gas which will cause voids in thesurface layer.

The sintering atmosphere is selected according to the nature of glasscomponents. Air, nitrogen or a mixture thereof is preferably used. Inorder to reduce unevenness in baked condition and in color tone of thefinal sintered AlN ceramics, it is advantageous to reduce the watercontent in the sintering atmosphere gas. The water content in the gascan be controlled by monitoring the dew point of a gas introduced. Inparticular, sintered AlN ceramics having reduced unevenness in bakedcondition and in color tone can be obtained under a reduced watercontent of the atmosphere gas at a dew point of lower than −30° C.

The present invention will be described in more detail with referring tonone-limitative Examples to which the present invention should not belimited.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the Examples, atomic ratios of Y, Ca and B were determined by aninductively coupled plasma emission spectroscopy (ICP analysis), thecontent of C was determined by LECO method proposed by LECO Corporationin USA and the content of O was determined by the infrared absorptionmethod. The thermal conductivity was determined by the laser flashmethod. The particle size of AlN was determined by measuring the maximumsize of each particle in a sintered article of AlN at a cut or slitsection by a scanning electronic microscope (SEM).

EXAMPLE 1

A sintered article of AlN consisting of 0.79% by weight of Y, 0.02% byweight of Ca, 0.015% by weight of C, 0.03% by weight of B, 0.73% byweight of O and a balance of of AlN and less than 0.1% by weight ofinevitable impurities was prepared. This sintered article of AlN has athermal conductivity of 120 W/m.K. An average in the particle size of 30particles of AlN was 3.5 μm.

A paste for the surface layer was prepared by kneading 82% by weight ofAlN powder whose oxygen content was 1.0% by weight produced byreduction-nitration method with a balance consisting of nitrocelluloseresin and butylcarbitol.

A surface of the sintered article of AlN was polished by a diamondgrinding stone #500 and then the paste for surface layer was applied byscreen printing technique at an area of 100 mm×100 mm. After theresulting paste film was leveled, the surface layer was sintered at1,800° C. for 2 hours so that the surface layer was densified.

The surface roughness (Ra) of the resulting AlN ceramics was determinedby a surface roughness gauge (Japanese Norm: JIS B0601) to find thesurface roughness (Ra) of 0.1 μm. The surface layer was observed by SEMto find its thickness of 48 μm and no defect larger than 25 μm was onthe surface of the surface layer.

EXAMPLE 2

The same paste for surface layer as Example 1 was screen-printed on thesame sintered article of AlN as Example 1 at different thickness.Samples were sintered and evaluated by the same method as Example 1. Theresults are summarized in Table 1.

TABLE 1 Defects larger Thickness of Surface than 25 μm at the Surfacelayer Roughness (Ra) a surface of Sample No. (μm) (μm) the surface layer 1* 7 0.5 Exist 2 11 0.3 None 3 27 0.2 None 4 48 0.1 None 5 101 0.08None 6 153 0.07 None 7 200 0.1 None 8 246 0.3 None  9* 300 0.5 Exist*Comparative Example

EXAMPLE 3

A sintered article of AlN consisting of 2.36% by weight of Y, 0.55% byweight of Ca, 0.025% by weight of C, 0.05% by weight of B, 0.66% byweight of O and a balance consisting of AlN and less than 0.1% by weightof inevitable impurities was prepared. This sintered article possessed athermal conductivity of 255 W/m.K. An average in the particle size of 30particles of AlN was 13.8 μm.

The same surface layer as Example 1 was applied by the same method asExample 1 and then sintered. After the resulting AlN ceramic was groundby diamond powders having a particle size of 2 μm, the samples wereevaluated by the same method as Example 1.

The surface roughness (Ra) was 0.03 μm and no defect larger than 25 μmwas found on a surface of the surface layer.

EXAMPLE 4

A sintered article of AlN consisting of 1.33% by weight of Y, 0.005% byweight of Ca, 0.025% by weight of C, 0.5% by weight of Ti, 0.03% byweight of B, 0.91% by weight of O and a balance consisting of AlN andless than 0.1% by weight of inevitable impurities. This sintered articlepossesses a thermal conductivity of 185 W/m.K and had dimensions of 100mm×100 mm and a thickness of 2 mm. An average in the particle size of 30particles of AlN was 6.5 μm. Ti compound was black and had a particlesize of 0.6 μm.

A paste for the surface layer was prepared by kneading 100 parts byweight of AlN powder whose oxygen content was 1.2% by weight produced bydirect nitration technique and 2.5 parts by weight of Y₂O₃ powder havingan average particle size of 0.8 μm together with the same solvent andorganic binder as Example 1. The powder content of the resulting pastefor surface layer was 75% by weight.

The paste for surface layer was spinner-coated on a surface of asintered article of AlN. After leveling, the coated surface layer wassintered for 1 hour at 1,850° C. to densify the surface layer. Thesurface roughness (Ra) evaluated by the same method as Example 1 was0.08 μm and no defect larger than 25 μm was found on a surface of thesurface layer.

EXAMPLE 5

The same sintered article of AlN as Example 4 was prepared and the samepaste for surface layer as Example 4 was coated at different powdercontents and sintered. The coating conditions were adjusted so that thesurface layer after sintering has a thickness of 50 μm±3 μm. The resultsare summarized in Table 2.

TABLE 2 Powder Surface Defects larger Appearance Sample content CoatingRoughness than 25 μm of the surface No. (%) Method (Ra) (μm) on asurface layer  10* 45 Spinner 0.05 Exist Porous 11 50 Spinner 0.05 NoneFine 12 60 Spinner 0.07 None Fine 13 75 Spinner 0.08 None Fine 14 90Screen 0.1 None Fine print 15 95 Screen 0.2 None Fine print *ComparativeExample

EXAMPLE 6

The AlN ceramic obtained in Example 4 was polished by diamond grainshaving the particle size of 1.2 μm.

The surface roughness (Ra) determined the same method as Example 1 was0.015 μm and no defect larger than 3 μm was found on a surface of thesurface layer.

EXAMPLE 7

A sintered article of AlN consisting of 0.79% by weight of Y, 0.02% byweight of Ca, 0.015% by weight of C, 0.03% by weight of B, 0.73% byweight of O and a balance of of AlN and less than 0.1% by weight ofinevitable impurities was prepared. This sintered article of AlN has athermal conductivity of 120 W/m.K. An average in the particle size of 30particles of AlN was 3.5 μm.

A paste for the surface layer was prepared by kneading 82% by weight ofoxide glass powder containing 25% by weight of Si, 8% by weight of Al,7% by wight of B and very small amounts of Sn, Mg, Fe, V and Cr with abalance consisting of nitrocellulose resin and butylcarbitol. Thethermal expansion coefficient of the oxide glass used was 3.7 ppm/°C.

A surface of the sintered article of AlN was polished by a diamondgrinding stone #500 and then the paste for surface layer was applied byscreen printing technique at an area of 100 mm×100 mm. After theresulting paste film was leveled, the surface layer was sintered at 800°C. for 30 minutes so that the surface layer was densified.

The surface roughness (Ra) of the resulting AlN ceramics was determinedby a surface roughness gauge (Japanese Norm: JIS B0601) to find thesurface roughness (Ra) of 0.06 μm. The surface layer was observed by SEMto find its thickness of 48 μm and no defect larger than 25 μm was onthe surface of the surface layer.

EXAMPLE 8

The same paste for surface layer as Example 7 was screen-printed on thesame sintered article of AlN as Example 7 at different thickness.Samples were sintered and evaluated by the same method as Example 7. Theresults are summarized in Table 3.

TABLE 3 Defects larger Thickness of Surface than 25 μm at the Surfacelayer Roughness (Ra) a surface of Sample No. (μm) (μm) the surface layer 1* 6 0.5 Exist 2 11 0.3 None 3 24 0.1 None 4 50 0.06 None 5 99 0.05None 6 151 0.08 None 7 202 0.2 None 8 247 0.3 None  9* 300 0.5 Exist*Comparative Example

EXAMPLE 9

A sintered article of AlN consisting of 2.36% by weight of Y, 0.55% byweight of Ca, 0.025% by weight of C, 0.05% by weight of B, 0.66% byweight of O and a balance consisting of AlN and less than 0.1% by weightof inevitable impurities was prepared. This sintered article possessed athermal conductivity of 255 W/m.K. An average in the particle size of 30particles of AlN was 13.8 μm.

The same surface layer as Example 7 was applied by the same method asExample 7 and then sintered. After the resulting AlN ceramic was groundby diamond powders having a particle size of 1.5 μm, the samples wereevaluated by the same method as Example 7.

The surface roughness (Ra) was 0.03 μm and no defect larger than 25 μmwas found on a surface of the surface layer.

EXAMPLE 10

A sintered article of AlN consisting of 1.33% by weight of Y, 0.005% byweight of Ca, 0.025% by weight of C, 0.5% by weight of Ti, 0.03% byweight of B, 0.91% by weight of O and a balance consisting of AlN andless than 0.1% by weight of inevitable impurities. This sintered articlepossesses a thermal conductivity of 185 W/m.K and had dimensions of 100mm×100 mm and a thickness of 2 mm. An average in the particle size of 30particles of AlN was 6.5 μm. Ti compound was black and had a particlesize of 0.6 μm.

A paste for the surface layer was prepared by kneading 75% by weight ofoxide glass powder containing 22% by weight of Si, 11% by weight of Al,5% by weight of Ca, 1% by weight of Zr and very small amounts, Fe, Bi, Band P (a balance consists mainly of oxygen) with nitrocellulose resinand butylcarbitol acetate. The oxide glass used has a thermal expansioncoefficient of 5.5 ppm/°C.

The paste for surface layer was spinner-coated on a surface of asintered article of AlN. After leveling, the coated surface layer wassintered for 1 hour at 1,050° C. to densify the surface layer.

The surface layer has a thickness of 48 μm. The surface roughness (Ra)evaluated by the same method as Example 7 was 0.05 μm and no defectlarger than 25 μm was found on a surface of the surface layer.

EXAMPLE 11

The same sintered article of AlN as Example 10 was prepared and the samepaste for surface layer as Example 10 was coated at different powdercontents and sintered. The coating conditions were adjusted so that thesurface layer after sintering has a thickness of 50 μm±3 μm. The resultsare summarized in Table 4.

TABLE 4 Powder Surface Defects larger Appearance Sample content CoatingRoughness than 25 μm of the surface No. (%) Method (Ra) (μm) on asurface layer  10* 45 Spinner 0.08 Exist Porous 11 50 Spinner 0.04 NoneFine & smooth 12 60 Spinner 0.04 None Fine & smooth 13 75 Spinner 0.05None Fine & smooth 14 90 Screen 0.08 None Fine & printing smooth 15 95Screen 0.1 None Fine & printing smooth *Comparative Example

EXAMPLE 12

The AlN ceramic obtained in Example 10 was polished by diamond abrasivegrains having the particle size of 1 μm.

The surface roughness (Ra) determined the same method as Example 7 was0.012 μm and no defect larger than 3 μm was found on a surface of thesurface layer.

EXAMPLE 13

The procedure of Example 7 was repeated but the sintered article waschanged. Namely, a sintered article was oxidation-treated.

A sintered article of AlN consisting of 0.95% by weight of Y, 0.02% byweight of Ca, 0.033% by weight of C, 0.05% by weight of B, 0.85% byweight of O, 1.03% by weight of W and a balance of of AlN and less than0.1% by weight of inevitable impurities was prepared. This sinteredarticle of AlN has a thermal conductivity of 165 W/m.K. An average inthe particle size of 30 particles of AlN was 3.8 μm.

This sintered article of AlN was oxidized in an atmosphere of a mixedgas consisting of 5% of oxygen and 95% of nitrogen at 1,050° C. for 1four. After the oxidation treatment, a treated surface of the sinteredarticle of AlN was polished by a diamond whetstone #500.

Then, the same paste for surface layer as Example 7 was screen-printedon the treated surface and was sintered by the same procedure as Example17.

The surface roughness (Ra) of the resulting sintered article of AlN was0.06 μm. The surface layer observed by SEM had a thickness of 50 μm andno defect larger than 10 μm was found on the surface.

EXAMPLE 14

The same procedure as Example 13 was repeated except that the sinteredarticle of AlN was oxidized in air at 750° C. for 2 hours.

The surface roughness (Ra) of the resulting sintered article of AlN was0.05 μm. The surface layer observed by SEM had a thickness of 52 μm andno defect larger than 10 μm was found on the surface.

EXAMPLE 15

On a surface of a AlN ceramic plate having a dimension of 100 mm×100 mmand a thickness of 3.0 mm, a first oxide glass paste having a softeningpoint of 880° C. and a thermal expansion coefficient of 4 ppm/°C. wascoated by screen-printing technique and was sintered in air at 1,000° C.for 2 hours to obtain a sintered first oxide glass layer having athickness of about 6 μm.

Then, a second oxide glass paste having a softening point of 800° C. anda thermal expansion coefficient of 3.5 ppm/°C. was coated on the firstoxide glass layer by screen-printing and was sintered in air at 900° C.for 1 hour to obtain a sintered second oxide glass layer having athickness of about 5 μm.

Finally, a third oxide glass paste having a softening point of 700° C.and a thermal expansion coefficient of 3.8 ppm/°C. was coated on thesecond oxide glass layer by screen-printing and was sintered in air at800° C. for 30 minutes to obtain a sintered second oxide glass layerhaving a thickness of about 7 μm.

The resulting sample was observed by an optical microscope (×400) tofind no defect on its outer surface layer. The surface roughness (Ra) ofthe outer surface layer was 0.02 μm.

What is claimed is:
 1. A method for producing aluminium nitride ceramichaving a dense smooth surface layer possessing a surface roughness Ra oflower than 0.3 μm and possessing no defect larger than 25 μm on itssurface, comprising (i) providing a sintered article of aluminiumnitride and then applying at least one layer of a paste of oxide glassthat contains no Na, no K, no Rb, and no Pb for the surface layerdirectly onto a surface of the sintered article of aluminium nitride andsintering said paste, or (ii) providing a preform of aluminium nitridewhich is not yet sintered and then applying at least one layer of apaste of oxide glass that contains no Na, no K, no Rb, and no Pb forsaid surface layer directly onto a surface of the preform and sinteringthe paste and the preform, wherein the paste is applied such that thefinal surface layer has a thickness between 10 μm and 250 μm aftersintering, thereby producing an aluminium nitride ceramic having a densesmooth surface layer possessing a surface roughness Ra of lower than 0.3μm and possessing no defect larger than 25 μm on its surface, andwherein the sintered aluminium nitride (1) comprises aluminate andaluminium oxide-nitride in which the oxygen content after an amount ofoxygen converted to aluminates is deducted from the total oxygen in thesintered article of aluminium nitride is less than 2% by weight, (2) thecarbon content in the sintered aluminium nitride substrate is in therange of 0.005 to 0.5% by weight, and (3) the boron content in thesintered aluminium nitride substrate is less than 1% by weight.
 2. Amethod for producing aluminium nitride ceramic having a dense smoothsurface layer possessing a surface roughness Ra of lower than 0.3 μm andpossessing no defect larger than 25 μm on its surface, comprisingproviding a preform of aluminium nitride which is not yet sintered andthen applying at least one layer of a paste of oxide glass that containsno Na, no K, no Rb, and no Pb for said surface layer directly onto asurface of the preform and sintering the paste and the preform, whereinthe paste is applied such that the final surface layer has a thicknessbetween 10 μm and 250 μm after sintering, thereby producing an aluminiumnitride ceramic having a dense smooth surface layer possessing a surfaceroughness Ra of lower than 0.3 μm and possessing no defect larger than25 μm on its surface.
 3. The method set forth in claim 1 or 2, wherein,after sintering, an outer surface of the resulting surface layer ispolished.
 4. The method set forth in claim 1 or 2, wherein said pastecontains more than 50% by weight of oxide glass powder.
 5. The methodset forth in claim 4, wherein a first oxide glass paste is applied ontosaid sintered article of AlN or preform and sintered and then anotheroxide glass paste is applied to the first oxide glass paste andsintered.
 6. The method set forth in claim 4, wherein all of the oxideglass paste layers are sintered simultaneously.
 7. The method set forthin claim 4, wherein the oxide glass powder has a particle size less than50 μm.
 8. The method set forth in claim 4, wherein the layers aresintered separately.
 9. The method set forth in claim 8, wherein eachlayer has a thickness of 1 to 100 μm, such that the final surface layerhas a thickness between 10 and 250 μm.
 10. The method set forth in claim4, wherein a surface of said sintered article or preform of aluminumnitride is oxidized before said sintered article or preform is coatedwith said paste of oxide glass.
 11. The method set forth in claim 10,wherein said oxidation is carried out at a temperature between 200 and1,300° C.
 12. The method set forth in claim 1 or 2, wherein a firstlayer of oxide glass is applied onto the sintered article of AlN orpreform and sintered and then a second layer of oxide glass is appliedonto the sintered article of AlN or preform and sintered.
 13. The methodset forth in claim 1 or 2, wherein each layer of oxide glass is polishedafter the sintering.
 14. The method set forth in 1 or 2, wherein aplurality of oxide glass layers are applied in which the softening pointof a layer closer to the substrate is lower than the layer formed overit.
 15. The method set forth in claim 1 or 2, wherein the paste of oxideglass is applied as a single layer that has a thickness between 10 μmand 250 μm after sintering.